0-RTT
Zero Round-Trip Time
A TLS 1.3 shortcut that lets a browser send its request in the very first packet to a server it has spoken to before, saving a full round trip.
In URL
Glossary
Every acronym that appears anywhere in the guides, with what it stands for and what it actually means. Search it, filter it by topic, or link straight to a term.
251 terms
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2G / 3G / 4G / 5G
Second to fifth generation mobile
The generations of mobile network. Each one is a different radio standard, not just a faster version of the last; 5G is the current one.
In 5G
802.11
IEEE 802.11
The family of standards that defines Wi-Fi. The letters after it mark the generation — 802.11n, ac, ax — which the Wi-Fi Alliance now renames Wi-Fi 4, 5 and 6.
In Wi-Fi
802.1X
IEEE 802.1X
Port-based network access control: the device must prove who it is to an authentication server before the network will carry any of its traffic. The basis of enterprise Wi-Fi logins.
In Wi-Fi
A
A2DP
Advanced Audio Distribution Profile
The Bluetooth profile that carries stereo music to headphones or a speaker. One-way, and separate from the profile used for calls.
In Bluetooth
AAAA
Quad-A record
The DNS record type that holds an IPv6 address, the way an A record holds an IPv4 one.
In URL
AAC
Advanced Audio Coding
A compression format that sounds better than SBC at the same bitrate. Widely used by Apple devices for Bluetooth audio.
In Bluetooth
ACK
Acknowledgement
A short message saying "I received that". Wi-Fi acknowledges every frame; TCP acknowledges every run of bytes. No acknowledgement means the sender tries again.
AHCI
Advanced Host Controller Interface
The command set SATA uses. It was designed around a mechanical arm, with one queue 32 commands deep, which became the bottleneck once drives had no moving parts.
In Hard drives SSDs
A-MPDU
Aggregated MAC Protocol Data Unit
Many Wi-Fi frames bundled into one transmission so the airtime overhead is paid once instead of dozens of times.
In Wi-Fi
ANonce
Authenticator Nonce
The random number the access point contributes to the four-way handshake. A nonce is a number used once, so the same password never produces the same key twice.
In Wi-Fi
AP
Access Point
The Wi-Fi radio that devices connect to — in a home it is built into the router, in an office it is a separate ceiling unit.
In Wi-Fi
aptX
aptX
Qualcomm’s family of Bluetooth audio codecs, common on Android. The Low Latency variant cuts delay to around 40 ms, which matters for video and games.
In Bluetooth
ARP
Address Resolution Protocol
How a device finds the MAC address that goes with an IP address on the local network. It shouts "who has 192.168.1.1?" and waits for the owner to answer.
In Wi-Fi
ASIO
Audio Stream Input/Output
A driver standard that bypasses the operating system’s normal audio path for lower latency. Common on audio interfaces used for recording.
In USB
ATM
Automated Teller Machine
A cash machine. It is stricter than a shop terminal at every step, because it dispenses physical banknotes and there is no chargeback for those.
In Cards
Auracast
Auracast broadcast audio
Bluetooth audio broadcast to unlimited receivers rather than one device — silent TVs in a gym, cinema audio description, or a lecture microphone going straight to hearing aids.
In Bluetooth
AVRCP
Audio/Video Remote Control Profile
The profile behind play, pause, skip and volume buttons on a Bluetooth device.
In Bluetooth
B
BCD
Binary-Coded Decimal
Storing each decimal digit in its own four bits rather than converting the whole number to binary. It wastes space and guarantees 0.1 + 0.2 is exactly 0.3.
In Calculators
BGA
Ball Grid Array
Storage soldered directly to the board, as in phones, tablets and thin laptops. Compact, and not replaceable.
In SSDs
BIN
Bank Identification Number
The older name for the IIN, still in common use.
In Cards
BIOS
Basic Input/Output System
The original PC firmware, limited to a 512-byte boot sector and about a megabyte of space. Replaced by UEFI, though people still say "the BIOS" for any firmware settings screen.
In Startup
BLE
Bluetooth Low Energy
A separate technology sharing the Bluetooth name. The radio is off almost all the time, waking briefly to send a few bytes, so a sensor can run for years on a coin cell.
In Bluetooth
BPSK
Binary Phase-Shift Keying
The simplest and most robust Wi-Fi modulation: one bit per symbol. Slow, but it survives a terrible signal.
In Wi-Fi
BR/EDR
Basic Rate / Enhanced Data Rate
The proper name for Classic Bluetooth — the continuous-stream technology used for audio and file transfer, as distinct from Low Energy.
In Bluetooth
BSS
Basic Service Set
One access point and the devices attached to it — a single cell of a Wi-Fi network.
In Wi-Fi
BSSID
Basic Service Set Identifier
The MAC address of one particular access point radio. Several access points share an SSID, but each has its own BSSID, which is how roaming can be observed.
In Wi-Fi
C
CA
Certificate Authority
An organisation your browser already trusts, which signs website certificates. The padlock means a CA vouched for the name on the certificate, nothing more.
In URL
CCMP
Counter Mode CBC-MAC Protocol
The AES-based encryption WPA2 uses to protect Wi-Fi frames.
In Wi-Fi
CDC
Communications Device Class
The USB class that emulates a classic serial port, used by serial adapters and some modems.
In USB
CDN
Content Delivery Network
A network of servers spread around the world that keep copies of a site's files, so the response comes from a nearby city rather than the origin server.
In URL
CGNAT
Carrier-Grade NAT
When a provider is so short of addresses that hundreds of customers share one public address. Usually invisible, until you try to host something or forward a port.
In Fibre
CNAME
Canonical Name
A DNS record that says "this name is really that name" — an alias that the resolver then has to look up in turn.
In URL
CORDIC
COordinate Rotation DIgital Computer
The algorithm behind sin, cos, log and square roots on a calculator. It rotates a vector through a fixed series of ever-smaller angles, each needing only a shift and an add.
In Calculators
CPU
Central Processing Unit
The processor. At power-on it starts executing at a fixed address called the reset vector, with no memory and no operating system yet.
In Startup
CR2032
CR2032 cell
The coin cell that powers a great many small devices. Its capacity is what makes a Low Energy sensor lasting several years possible.
In Bluetooth
CSS
Cascading Style Sheets
The language that describes how a web page looks. The browser must fetch and parse it before it can paint anything.
In URL
CSSOM
CSS Object Model
The browser's parsed, in-memory version of the stylesheets, combined with the DOM to work out what every element should look like.
In URL
CTS
Clear To Send
The reply to an RTS. The pair reserves the air for one device, which stops two stations that cannot hear each other from talking over one another.
In Wi-Fi
CVC
Card Verification Code
Mastercard’s name for the same three-digit code that Visa calls the CVV.
In Cards
CVV
Card Verification Value
The three digits on the back, computed from the card number and expiry with a key only the bank holds. Deliberately not stored in the chip or the stripe.
In Cards
D
DAN
Device Account Number
The substitute card number a phone wallet is given. It only works from that device, so a shop that is breached leaks something the bank can cancel on its own.
In Cards
dB
Decibel
A ratio on a logarithmic scale. Every 3 dB is double or half the power, and every 10 dB is ten times — which is why small dB numbers describe enormous differences.
dBm
Decibels relative to one milliwatt
The usual unit for Wi-Fi signal strength. It is negative in practice: -50 dBm is excellent, -70 dBm is workable, -80 dBm is trouble.
In Wi-Fi
dBW
Decibels relative to one watt
The unit used for satellite footprint maps. A higher dBW figure over your area means a smaller dish will do.
In Satellite
DCF77
DCF77 time signal
Germany’s long-wave time broadcast, receivable across much of Europe and used by radio-controlled clocks well beyond Germany.
In Timekeeping
DDR5
Double Data Rate 5
The current generation of system memory. The firmware has to train it — tuning timings until the chips read back correctly — before it can be used at all.
In Startup
DFS
Dynamic Frequency Selection
The rule that makes Wi-Fi vacate certain 5 GHz channels when it detects radar, because those channels are shared with weather and military systems.
In Wi-Fi
DHCP
Dynamic Host Configuration Protocol
How a device that has just joined a network is handed an IP address, a gateway and a DNS server, instead of being configured by hand.
DIFS
DCF Interframe Space
The longer gap a Wi-Fi device must wait before it may start transmitting. Shorter waits get priority, which is how acknowledgements jump the queue.
In Wi-Fi
DKIM
DomainKeys Identified Mail
A DNS-published signature that lets a receiving mail server check a message really came from the domain it claims.
In URL
DNS
Domain Name System
The internet's directory: it turns a name people can remember into the IP address a machine can route to. Nothing else can start until it answers.
DOCTYPE
Document Type Declaration
The first line of an HTML file. Its only real job now is to keep the browser in standards mode rather than an old compatibility mode.
In URL
DOM
Document Object Model
The browser's tree-shaped, in-memory model of the HTML. Scripts change the page by changing the DOM, not the original file.
In URL
DRAM
Dynamic Random-Access Memory
Fast volatile memory. An SSD uses a little of it to hold its address map; a drive without any has to look the map up from flash, which roughly doubles small-operation latency.
In SSDs
DST
Daylight Saving Time
Moving the clocks seasonally. It creates one hour that never happens in spring and one that happens twice in autumn — the source of a great many scheduling bugs.
In Timekeeping
DVB-S2
Digital Video Broadcasting — Satellite, 2nd generation
The standard that defines how a satellite TV signal is modulated and error-protected. Its efficiency is why a modern transponder carries far more channels than it used to.
In Satellite
DWPD
Drive Writes Per Day
The same endurance idea from the other direction: how many times the whole drive could be rewritten daily for its warranty period.
In SSDs
E
EAP
Extensible Authentication Protocol
The framework 802.1X uses to carry a login — a username and password, or a certificate — between the device and the authentication server.
In Wi-Fi
ECM
Entitlement Control Message
The constantly changing decryption key for a scrambled channel, encrypted and broadcast to everyone. Only a card that is entitled can unwrap it.
In Satellite
EFI
Extensible Firmware Interface
The interface UEFI implements. In practice you meet it as the \EFI folder on the boot partition, holding one bootloader file per operating system.
In Startup
EGNOS
European Geostationary Navigation Overlay Service
A European augmentation system that broadcasts correction data, sharpening GPS from metres to roughly a metre.
In GPS
EIRP
Effective Isotropic Radiated Power
How much signal power actually arrives at a given point on the ground. It decides the dish size you need, and it is fixed when the satellite is built.
In Satellite
EMM
Entitlement Management Message
A message addressed to one specific smart card, telling it which channels the subscription covers. Turning a package on or off is an EMM.
In Satellite
eMMC
embedded MultiMediaCard
Cheap soldered-down flash storage with a simple controller. Common in budget laptops and tablets, and far slower than an SSD.
In SSDs
EMV
Europay, Mastercard and Visa
The chip card standard. The chip performs a calculation only it can perform, over details unique to that one transaction, which is what ended mass card cloning.
In Cards
EPG
Electronic Programme Guide
The on-screen schedule. It arrives as just another stream of data packets alongside the picture and sound.
In Satellite
EPR
Extended Power Range
The USB Power Delivery extension reaching 240 watts, using voltages up to 48 V for large laptops and monitors.
In USB
eSIM
Embedded SIM
A SIM soldered into the phone and provisioned over the air, rather than a plastic card you swap. Same role, no tray.
In 5G
ESP
EFI System Partition
A small FAT partition the firmware can read without any operating system drivers. It holds the bootloaders.
In Startup
F
FAT
File Allocation Table
A deliberately simple filesystem. It is used for the EFI System Partition precisely because firmware can implement it in very little code.
In Startup
FCS
Frame Check Sequence
A checksum at the end of a Wi-Fi frame. If it does not match, the frame is discarded and never acknowledged, so the sender retransmits.
In Wi-Fi
FEC
Forward Error Correction
Deliberate redundancy added to a transmission so the receiver can repair errors without asking for anything to be resent — essential when the sender is 36,000 km away.
In Satellite
FTL
Flash Translation Layer
The map inside an SSD from the addresses the computer uses to the physical pages holding the data. It changes constantly, and the computer is never told.
In SSDs
FTTB
Fibre To The Building
Fibre to a block of flats or an office, with copper or ethernet handling the distribution inside.
In Fibre
FTTC
Fibre To The Cabinet
Fibre to a green street cabinet, then existing copper phone line for the last hundred metres or so. Sold as fibre; the speed depends on how far away you live.
In Fibre
FTTH
Fibre To The Home
The same thing as FTTP, named from the customer’s side rather than the network’s.
In Fibre
FTTP
Fibre To The Premises
Glass running the whole way to your building, with no copper anywhere in the path. Distance barely affects the speed.
In Fibre
G
GATT
Generic Attribute Profile
How a Low Energy device describes itself — a small self-describing structure of services and characteristics, rather than a fixed profile both ends must already know.
In Bluetooth
GCMP
Galois/Counter Mode Protocol
The encryption WPA3 and faster Wi-Fi use in place of CCMP. Stronger, and cheaper to run at high data rates.
In Wi-Fi
GDM
GNOME Display Manager
The Linux program that draws the login screen and starts your desktop session once you have authenticated.
In Startup
GET
HTTP GET
The request method meaning "give me this". It should never change anything on the server, which is why it is safe to repeat, cache and bookmark.
In URL
G.fast
G.fast
FTTC pushed harder, using a much wider band of frequencies on the copper. It delivers far more speed over far less distance — about 100 metres.
In Fibre
GHz
Gigahertz
A billion cycles per second. Wi-Fi uses 2.4, 5 and 6 GHz bands; satellite TV arrives in the Ku band around 10–12 GHz.
GLONASS
Globalnaya Navigatsionnaya Sputnikovaya Sistema
Russia's satellite navigation system. Most phones use it alongside GPS, which is why they can see far more satellites than GPS alone provides.
In GPS
GMR
Giant Magnetoresistance
The effect behind modern read heads: a material whose electrical resistance changes in a magnetic field. Discovered in 1988, Nobel Prize in 2007, in your drive within a decade.
In Hard drives
GMT
Greenwich Mean Time
The older astronomical standard UTC replaced for technical use. In everyday British usage it now means the winter offset, UTC+0.
In Timekeeping
GNSS
Global Navigation Satellite System
The umbrella term for all of them — GPS, GLONASS, Galileo and BeiDou. "GPS" in everyday speech usually means GNSS.
In GPS
GPON
Gigabit Passive Optical Network
The common form of PON: about 2.5 Gbps downstream shared across a splitter serving typically 32 properties.
In Fibre
GPS
Global Positioning System
The American satellite navigation system: about thirty-one satellites broadcasting the time, from which a receiver works out where it is. It only listens — it never transmits.
In GPS Timekeeping
GPU
Graphics Processing Unit
The chip that does the actual drawing. The browser hands it the finished layers to composite onto the screen.
In URL
GRUB
GRand Unified Bootloader
The bootloader most Linux systems use. It is what shows the menu of kernels and other operating systems.
In Startup
GTK
Group Temporal Key
The single shared key that encrypts broadcast and multicast Wi-Fi traffic, handed out during the four-way handshake.
In Wi-Fi
H
H.264
Advanced Video Coding
The video compression standard behind most HD broadcasting. Widely supported, and less efficient than HEVC.
In Satellite
HAMR
Heat-Assisted Magnetic Recording
Using a laser to heat a spot for about a nanosecond so it can be written, then letting it cool into stability. It is how drives keep growing past the density limit.
In Hard drives
HD
High Definition
Broadcast pictures at 1280x720 or 1920x1080, as opposed to the older standard-definition formats.
In Satellite
HDD
Hard Disk Drive
Storage built from spinning magnetic discs and a head that flies two nanometres above them. Slower than flash by a factor of hundreds, and several times cheaper per terabyte.
In Hard drives
HDMI
High-Definition Multimedia Interface
The single cable that carries picture, sound and control signals from the set-top box to the television.
HEAD
HTTP HEAD
A request for the headers only, with no body — used to check whether something has changed without downloading it.
In URL
HEVC
High Efficiency Video Coding
The successor to H.264, also called H.265. Roughly half the bitrate for the same picture, which is what makes 4K broadcasting practical.
In Satellite
HFC
Hybrid Fibre-Coax
Cable broadband: fibre to a local node, then shared coaxial cable to the homes around it.
In Fibre
HFP
Hands-Free Profile
The profile used during a call. It is two-way, so capacity is split between the earpiece and the microphone — which is why music quality collapses the moment a call connects.
In Bluetooth
HID
Human Interface Device
The USB class covering keyboards, mice and controllers. A device describes its own controls, which is why a racing wheel or foot pedal works with no driver.
HLC
Hybrid Logical Clock
A timestamp that combines a physical time with a counter, so values still look like real times to a human while guaranteeing a consistent order across machines.
In Timekeeping
HMB
Host Memory Buffer
A cheaper alternative to on-board DRAM, where the SSD borrows a little of the computer’s memory over PCIe to hold its map.
In SSDs
HSM
Hardware Security Module
Tamper-resistant hardware that performs cryptography without ever revealing its keys. A cash machine’s keypad is one; opening it destroys the keys inside.
In Cards
HSP
Headset Profile
An older, simpler version of HFP for basic headsets with a single earpiece and microphone.
In Bluetooth
HSTS
HTTP Strict Transport Security
A header telling the browser to only ever use HTTPS for this site from now on, so a plaintext request is never sent — not even the first redirect.
In URL
HTML
HyperText Markup Language
The language web pages are written in. The browser parses it into the DOM as it arrives, rather than waiting for the whole file.
In URL
HTTP
HyperText Transfer Protocol
The request-and-response protocol the web runs on: a method, a path, some headers, and a body coming back.
HTTP/2
HTTP version 2
A binary version of HTTP that multiplexes many requests over one connection, removing the queueing that made HTTP/1.1 slow.
In URL
HTTP/3
HTTP version 3
HTTP carried over QUIC and UDP instead of TCP, so one lost packet no longer stalls every other request on the connection.
In URL
HTTPS
HTTP Secure
HTTP inside a TLS tunnel. The protocol is unchanged; everything is simply encrypted and the server's identity is checked first.
In URL Timekeeping
I
IANA
Internet Assigned Numbers Authority
Among other things, it maintains the time zone database that says which rules apply where. Countries change those rules several times a year.
In Timekeeping
IC
Integrated Circuit
A chip. A laptop's power management IC brings up a dozen voltage rails in a precise order before the processor may start.
In Startup
IDE
Integrated Drive Electronics
The older name for PATA, from the idea of putting the controller on the drive itself rather than on a separate card.
In Hard drives
IIN
Issuer Identification Number
The first six to eight digits of a card number, identifying the bank and the network. It is how a checkout shows the right logo before you finish typing.
In Cards
IOPS
Input/Output Operations Per Second
How many separate reads or writes a drive handles each second. For everyday use this matters far more than the sequential speed on the box.
In SSDs
IP
Internet Protocol
The addressing and routing scheme the internet is built on. Every packet carries a source and destination IP address and nothing else about the conversation.
IPoE
IP over Ethernet
The alternative to PPPoE, where you are identified by which physical port your fibre lands on and your router simply asks for an address with DHCP.
In Fibre
IPv4
Internet Protocol version 4
The original addressing scheme, written as four numbers such as 93.184.216.34. There are not enough of them, which is why NAT exists.
IPv6
Internet Protocol version 6
The much larger address space, written in hexadecimal groups. Enough addresses that every device can have its own.
ISM
Industrial, Scientific and Medical band
Spectrum anyone may use without a licence. It is why Bluetooth and Wi-Fi are free to use worldwide, and why they share the band with microwave ovens.
In Bluetooth
ISP
Internet Service Provider
The company that carries your traffic to the rest of the internet, and usually runs the DNS resolver your router is handed by default.
In URL
K
L
LAN
Local Area Network
The network inside one building or home, as opposed to the internet beyond the router.
LBA
Logical Block Addressing
Addressing a drive by block number rather than by head, track and sector. The drive alone knows where a block physically lives, which lets it remap bad sectors invisibly.
In Hard drives
LC3
Low Complexity Communication Codec
The codec behind LE Audio. Better quality than SBC at roughly half the bitrate, and used in both directions so calls no longer degrade the sound.
In Bluetooth
LCD
Liquid Crystal Display
A display that emits no light of its own. It draws digits by blocking ambient light instead, which is why it costs almost nothing to run and needs a lamp to read.
In Calculators
LDAC
LDAC
Sony’s high-bitrate Bluetooth codec, reaching up to 990 kbps. The most data of any common codec, and the least tolerant of a crowded radio environment.
In Bluetooth
LED
Light-Emitting Diode
A component that produces light when current passes through it. Used in early calculator displays, and abandoned for LCDs because it drained batteries.
In Calculators
LNB
Low-Noise Block downconverter
The unit on the dish arm. It amplifies the very weak incoming signal and shifts it down to a frequency a coaxial cable can carry indoors.
In Satellite
LSASS
Local Security Authority Subsystem Service
The Windows process that checks your password and issues the tokens that decide what you are allowed to do.
In Startup
LVM
Logical Volume Manager
A layer between partitions and filesystems on Linux, letting volumes be resized and spanned across disks.
In Startup
M
M.2
M.2
The bare stick form factor, usually 22 × 80 mm. It may carry SATA or NVMe — the notches differ, and many slots accept only one.
In SSDs
MAC address
Media Access Control address
The hardware address of a network interface, written as six pairs of hexadecimal digits. Unique per radio, and the reason phones now randomise it.
In Wi-Fi
Mbps
Megabits per second
A million bits per second. Note bits, not bytes — divide by eight for megabytes.
MCS
Modulation and Coding Scheme
The index describing how densely data is packed into the radio signal. A clean link supports a high MCS; a noisy one forces the rate down.
In Wi-Fi
MHz
Megahertz
A million cycles per second. Wi-Fi channels are 20 to 160 MHz wide, and the width is one of the three things that sets the speed.
MIC
Message Integrity Check
Proof attached to a handshake message showing the sender holds the right key, and that nothing was altered in transit.
In Wi-Fi
MIMO
Multiple Input, Multiple Output
Using several antennas at each end to send independent streams of data at the same time on the same frequency.
MLC
Multi-Level Cell
Two bits per cell across four charge levels. Once the enthusiast standard, now largely displaced by denser arrangements.
In SSDs
MPDU
MAC Protocol Data Unit
One Wi-Fi frame as it goes over the air, headers and all.
In Wi-Fi
MSF
MSF time signal
The UK’s long-wave radio time broadcast, from Anthorn in Cumbria. What a “radio controlled” wall clock is listening to.
In Timekeeping
MTU
Maximum Transmission Unit
The largest packet that fits. PPPoE drops it from 1500 to 1492 bytes, and when something mishandles that, small requests work while large ones hang forever.
In Fibre
MU-MIMO
Multi-User MIMO
MIMO aimed at several devices at once, so one transmission can serve more than one client instead of taking turns.
In Wi-Fi
MX
Mail Exchange
The DNS record that says which server handles email for a domain.
In URL
N
NAND
NAND flash
The memory in every SSD and memory card. A bit is a handful of electrons trapped on an insulated island, which is why it survives with no power at all.
In SSDs
NAT
Network Address Translation
The router's trick of making every device behind it share one public IP address, rewriting addresses and ports as traffic passes through.
NCQ
Native Command Queuing
Letting the drive reorder pending requests using knowledge only it has — where the head is and exactly where the platter has rotated to. Like a lift choosing its own order.
In Hard drives SSDs
NFC
Near-Field Communication
Short-range radio at 13.56 MHz, powered by the reader’s own magnetic field. Its few-centimetre range is a physical limit, not a rule.
NR
New Radio
The radio standard that makes 5G 5G, as distinct from the core network behind it.
In 5G
NS
Name Server
The DNS record naming which servers are authoritative for a domain — the ones with the real answers.
In URL
NTP
Network Time Protocol
How almost every device corrects its clock. It measures how long its own question took to travel, subtracts that, and then nudges the clock gently rather than jumping it.
In Timekeeping
NTS
Network Time Security
Cryptographic authentication for NTP, so a machine cannot be fed a deliberately wrong time — without needing a correct clock in order to check.
In Timekeeping
NVMe
Non-Volatile Memory Express
The command set built for flash rather than for a moving arm. It allows 65,535 queues of 65,535 commands, against SATA’s single queue of 32.
In Hard drives SSDs
NXDOMAIN
Non-Existent Domain
The DNS answer meaning the name simply does not exist, as opposed to existing with no address.
In URL
O
OFDMA
Orthogonal Frequency-Division Multiple Access
Splitting one Wi-Fi channel into subcarriers that can be handed to different devices at the same instant, instead of making each wait its turn.
In Wi-Fi
OLT
Optical Line Terminal
The operator’s equipment in the exchange. It broadcasts downstream to every home on the splitter and tells each one precisely when it may transmit back.
In Fibre
ONT
Optical Network Terminal
The box on your wall where the glass ends. It converts light to ethernet and back, belongs to the network operator, and has no idea what the internet is.
In Fibre
OPTIONS
HTTP OPTIONS
A request asking what a server will allow. Browsers send it automatically as the preflight check before certain cross-origin requests.
In URL
OTG
On-The-Go
The extension that lets a device act as a USB host rather than only a peripheral — how a phone reads a flash drive.
In USB
P
PAM
Pluggable Authentication Modules
The Linux framework that decides how a login is checked — password, fingerprint, smart card — without every program implementing it itself.
In Startup
PAN
Primary Account Number
The long number on the front of a card. It identifies the issuing bank, then the account, and checks itself with a final digit.
In Cards
PATA
Parallel ATA
The wide ribbon cable SATA replaced. It sent 16 bits at once, which stopped working as speeds rose and the bits no longer arrived together.
In Hard drives
PBKDF2
Password-Based Key Derivation Function 2
The deliberately slow calculation that turns a Wi-Fi password into the PMK. Slowness is the point: it makes guessing expensive.
In Wi-Fi
PCIe
Peripheral Component Interconnect Express
The high-speed bus inside a computer. NVMe drives, graphics cards and Thunderbolt devices all speak it.
In USB Hard drives SSDs
PCR
Platform Configuration Register
A register in the TPM holding a running hash of everything loaded so far. It can be extended but never rewritten, which is what makes measured boot trustworthy.
In Startup
PD
Power Delivery
The negotiation that lets USB carry more than 15 watts. The charger advertises a menu of voltages, the device picks one, and the cable gets a vote.
In USB
PHY
Physical layer
The radio itself. The PHY rate is the headline speed negotiated between two radios — always well above the throughput you actually get.
In Wi-Fi
PID
Packet Identifier
The number on every satellite transport packet saying which stream it belongs to — one PID for picture, another for sound, another for subtitles.
In Satellite
PID
Process ID
The number the operating system gives each running process. PID 1 is the first one started, and the ancestor of everything else.
In Startup
PIN
Personal Identification Number
The code proving you are entitled to use the card. At a shop it is often checked inside the chip itself; at a cash machine it always goes to the bank.
In Cards
PLC
Penta-Level Cell
Five bits per cell, thirty-two levels. Still in development, and aimed at archival storage where writes are rare.
In SSDs
PMK
Pairwise Master Key
The key derived from the Wi-Fi password and network name. It never crosses the air — both ends calculate it independently.
In Wi-Fi
PMR
Perpendicular Magnetic Recording
Standing the magnetic regions on end rather than laying them flat. Adjacent regions reinforce instead of fighting, which gave roughly a tenfold density gain around 2006.
In Hard drives
PON
Passive Optical Network
A network where one fibre from the exchange is divided among many homes by an unpowered glass splitter. Nothing between the two ends needs electricity.
In Fibre
POST
Power-On Self-Test
The firmware's check that the essential hardware is present and working before it tries to boot anything.
In Startup
POST
HTTP POST
The request method meaning "here is something, do it". Repeating it repeats the action — hence the warning about pressing back after submitting a form.
In URL
ppm
parts per million
How clock error is quoted. A crystal 20 ppm fast gains about 1.7 seconds a day, which is roughly ten minutes a year if nothing corrects it.
In Timekeeping
PPP
Point-to-Point Protocol
A login and session protocol designed for dial-up modems. Broadband kept the part that handled authentication and billing.
In Fibre
PPPoE
PPP over Ethernet
How most providers check a line belongs to a paying customer. Your router logs in automatically, and the session carries your speed tier and public address.
In Fibre
PTK
Pairwise Transient Key
The session key protecting traffic between one device and one access point, built during the four-way handshake and thrown away afterwards.
In Wi-Fi
PTP
Precision Time Protocol
Timekeeping for when milliseconds are too coarse. Timestamps are taken in the network card itself and every switch declares how long it held the packet, reaching sub-microsecond accuracy.
In Timekeeping
PXE
Preboot Execution Environment
Booting from the network: the firmware fetches a bootloader over the LAN instead of from a local disk. Pronounced "pixie".
In Startup
Q
QAM
Quadrature Amplitude Modulation
Encoding data as distinct combinations of amplitude and phase. 4096-QAM packs twelve bits into one symbol, but needs a near-perfect signal to tell the positions apart.
In Wi-Fi
QLC
Quad-Level Cell
Four bits per cell across sixteen levels. Cheap and dense, with narrow margins — slower to write and rated for far fewer erase cycles.
In SSDs
QUIC
A transport protocol built on UDP that carries HTTP/3. Not an acronym any more — the name simply stuck.
In URL
R
RADIUS
Remote Authentication Dial-In User Service
The server that enterprise Wi-Fi asks whether a username and password are valid. The name is a relic of dial-up.
In Wi-Fi
RAID
Redundant Array of Independent Disks
Combining several drives so they behave as one, for speed, for redundancy, or both.
In Startup
RAM
Random Access Memory
The machine's working memory. At power-on there is none usable, so the processor runs from cache configured as RAM until the memory is trained.
In Startup
ROM
Read-Only Memory
Memory that cannot be written in normal operation. The first instructions after reset come from something of this kind.
rpm
revolutions per minute
How fast the platters spin. It sets rotational latency directly: at 7,200 rpm the average wait for a sector to come round is 4.2 milliseconds.
In Hard drives
RSSI
Received Signal Strength Indicator
The measure of how strong a received Wi-Fi signal is, usually reported in dBm. Strength alone does not mean a good link — noise matters just as much.
In Wi-Fi
RTC
Real-Time Clock
A tiny battery-backed clock on the motherboard whose only job is to remember roughly what time it was while the machine was switched off. The least accurate clock in the building.
In Timekeeping
RTS
Request To Send
A short frame asking to reserve the air before a long transmission, answered by a CTS.
In Wi-Fi
RTT
Round-Trip Time
How long a packet takes to get there and back. Every extra round trip in a handshake costs one of these, which is why distance is felt as latency.
In URL
S
SAE
Simultaneous Authentication of Equals
The WPA3 handshake that replaced WPA2's. Capturing it gives an attacker nothing to take away and guess at offline.
In Wi-Fi
SATA
Serial ATA
The interface most drives plug into. One very fast serial lane replaced sixteen parallel ones, because skew and crosstalk cap how fast parallel wiring can go.
In Hard drives SSDs
SBC
Subband Coding
The mandatory Bluetooth audio codec. Every device supports it, and everything else is an optional improvement negotiated when the two ends connect.
In Bluetooth
SCA
Strong Customer Authentication
The European rule requiring periodic verification — which is why a contactless card demands its PIN after a number of taps or a certain total spend.
In Cards
SCSI
Small Computer System Interface
An older disk command set. USB mass storage is essentially SCSI commands carried over USB, which is why any flash drive works everywhere.
In USB
SDDM
Simple Desktop Display Manager
The login screen used by KDE and several other Linux desktops.
In Startup
SIFS
Short Interframe Space
The shortest gap between Wi-Fi transmissions. Acknowledgements use it, so they always get in ahead of any new traffic.
In Wi-Fi
SIM
Subscriber Identity Module
The card holding the secret key that proves to the network which subscriber you are. The key itself never leaves the card.
In 5G
SLC
Single-Level Cell
One bit per flash cell, with two charge levels and enormous margins. The fastest and most durable arrangement, and the most expensive per gigabyte.
In SSDs
SMART
Self-Monitoring, Analysis and Reporting Technology
Health data a drive reports about itself. Percentage used and reserved-block counts give genuine warning long before a failure.
In Hard drives SSDs
SMR
Shingled Magnetic Recording
Overlapping tracks like roof tiles for about 25% more capacity. Rewriting one track destroys its neighbours, so the drive must rewrite a whole band — fine for archives, poor for anything else.
In Hard drives
SNI
Server Name Indication
The TLS extension that tells the server which site is wanted, so one IP address can host many certificates. Historically sent in the clear.
In URL
SNonce
Supplicant Nonce
The random number the client contributes to the four-way handshake, paired with the access point's ANonce.
In Wi-Fi
SNR
Signal-to-Noise Ratio
How far the signal stands above the background noise, in dB. It, rather than raw signal strength, sets the speed a link can sustain.
In Wi-Fi
SPF
Sender Policy Framework
A DNS record listing which servers may send mail for a domain.
In URL
SPI
Serial Peripheral Interface
The simple bus connecting the processor to the flash chip holding the firmware — the first thing it reads after reset.
In Startup
SSD
Solid-State Drive
Storage with no moving parts. Fast enough that the boot delay you notice is almost never the disk any more.
SSID
Service Set Identifier
The network name you pick from a list. It is not an address and it is not unique — anything can broadcast any name.
In Wi-Fi
SYN
Synchronise
The first packet of a TCP connection, answered by a SYN-ACK and then an ACK. That is the three-way handshake, and it costs a round trip before any data moves.
In URL
T
TAI
International Atomic Time
The weighted average of around 450 atomic clocks worldwide. It never stops or skips, which is exactly why it slowly drifts away from daylight.
In Timekeeping
TBW
Terabytes Written
An endurance rating. A typical 1 TB consumer drive is rated around 600 TB, which is about 330 GB every day for five years.
In SSDs
TCP
Transmission Control Protocol
The protocol that turns unreliable packets into an ordered, reliable stream, by numbering everything and resending what is not acknowledged.
TLC
Triple-Level Cell
Three bits per cell across eight charge levels. What almost every consumer SSD uses, balancing cost against endurance.
In SSDs
TLS
Transport Layer Security
The encryption layer under HTTPS. It proves the server's identity with a certificate and then encrypts everything that follows. Formerly called SSL.
In Wi-Fi URL Timekeeping
TOTP
Time-based One-Time Password
The six-digit code in an authenticator app. It is derived from a shared secret and the current time, which is why the code fails if either clock drifts too far.
In Timekeeping
TPM
Trusted Platform Module
A separate chip that records what was loaded during boot and will only release the disk encryption key if those measurements match.
In Startup
TRIM
TRIM command
How the operating system tells a drive which blocks hold deleted files. Without it the drive keeps carefully preserving data nobody wants, for as long as it lives.
In SSDs
TTFB
Time To First Byte
How long from sending the request to the first byte of the response arriving. It separates network and server delay from rendering delay.
In URL
TTL
Time To Live
How long a cached DNS answer may be reused before it must be looked up again. A low TTL is what makes a planned change take effect quickly.
In URL
TXT
Text record
A DNS record holding free-form text. In practice it carries verification strings and mail policies such as SPF and DKIM.
In URL
Type-A
USB Type-A
The original rectangular USB plug. Unlike USB-C, its shape guaranteed what it could do.
In USB
U
U.2
U.2
A 2.5-inch form factor carrying NVMe over PCIe, hot-swappable and used in servers.
In SSDs
UDP
User Datagram Protocol
Send-and-hope: no connection, no ordering, no retransmission. Useful when a late packet is worth less than a lost one, and the base QUIC builds on.
In URL Timekeeping
UEFI
Unified Extensible Firmware Interface
The modern replacement for the BIOS. It can read filesystems, has drivers, a shell and its own variable storage, and it is what holds the boot order.
In Startup
URL
Uniform Resource Locator
A web address: the scheme, the host, the path and anything after it. It names both where a thing is and how to ask for it.
In URL
USB
Universal Serial Bus
The general-purpose connector for peripherals and, at boot time, one of the devices the firmware can be told to start from.
USB4
USB4
The 40 Gbps generation, built on Thunderbolt and carried over USB-C. Capable of tunnelling PCI Express and DisplayPort alongside USB traffic.
In USB
USB-C
USB Type-C
The reversible oval connector. It is a shape, not a capability — the same port may carry USB 2.0 and nothing else, or 40 Gbps plus video plus 240 watts.
In USB
UT1
Universal Time 1
Time according to the actual orientation of the Earth, measured by pointing radio telescopes at distant quasars. It matches the Sun, and it is irregular.
In Timekeeping
UTC
Coordinated Universal Time
The time standard everything on the internet uses. It ticks at the atomic rate but has occasionally had a second inserted to keep pace with the Earth’s rotation.
In Timekeeping
UVC
USB Video Class
The standard that makes any webcam work on any operating system without installing anything.
In USB
V
VID
Vendor ID
A 16-bit number identifying the manufacturer of a USB device, allocated for a substantial fee — which is why cheap hardware often ships with a borrowed one.
In USB
VLAN
Virtual LAN
One physical network divided into several logical ones, so guest and corporate traffic can share the same cables and stay separate.
In Wi-Fi
VPN
Virtual Private Network
An encrypted tunnel carrying your traffic to somewhere else before it reaches the internet. It moves who can see your traffic; it does not remove the need for HTTPS.
W
WAF
Write Amplification Factor
How much flash is actually written compared with what the computer asked to save. A nearly full drive can exceed ten, meaning every megabyte costs ten megabytes of wear.
In SSDs
WAN
Wide Area Network
The side of your router that faces the provider, as opposed to the LAN side facing your own devices.
In Fibre
Wi-Fi
Not an acronym and not short for anything — a trademark the Wi-Fi Alliance invented because "IEEE 802.11b Direct Sequence" was never going to sell.
Wi-Fi 6E
Wi-Fi 6 Extended
Wi-Fi 6 given access to the new 6 GHz band: the same radio standard with far more clean spectrum to use.
In Wi-Fi
WPA2
Wi-Fi Protected Access 2
The long-standing Wi-Fi security standard, using a four-way handshake and CCMP encryption. Its handshake can be captured and guessed at offline.
In Wi-Fi
WPA3
Wi-Fi Protected Access 3
The current standard. It replaces the handshake with SAE, so capturing it no longer gives an attacker anything worth taking away.
In Wi-Fi
X
XGS-PON
10 Gigabit Symmetrical PON
The upgrade from GPON, offering 10 Gbps in both directions. It uses different wavelengths, so it can run on the same glass while customers are migrated.
In Fibre
XOR
Exclusive OR
A logic gate that outputs 1 when exactly one input is 1. Paired with an AND gate it adds two bits, which is the foundation of all arithmetic in hardware.
In Calculators
Z
ZBR
Zone Bit Recording
Packing more sectors into the longer outer tracks. It is why the same drive is often twice as fast at the start of its address range as at the end.
In Hard drives